Screen-printing apparatus in which a body is formed with a distribution duct for a flowable medium, which can be a viscous medium such as an ink, a dyestuff or another flowable coloring substance, effects relative movement of the surface or substrate to be printed and a printing station at which the body provided with this duct is located. A screen defiring the pattern or template to be printed onto the substrate likewise can be moved across this body which can have a multiplicity of openings juxtaposed with the surface and, of course, the portion of the screen interposed between this body and the surface.
The flowable medium can be wiped through the screen, where the openings of the latter are not obstructed by a patterning, or the flowable medium can be forced through the screen by a roller. The wiping action can be effected by a blade which generally is flexible and extends the full width of the region to which the flowable medium is to be applied and can be located downstream of the openings formed in the body and connected to the duct which feed the flowable medium to the screen. A bank of this material can accumulate upstream of the doctor blade or wiper or the applicator roller.
In practice it is desirable to be able, with such a screen printing apparatus, to effect a rapid color change, i.e. minimize the downtime or standstill of the apparatus which has hitherto been necessary to effect a color change, i.e. a changeover between the application of one flowable medium, e.g. one color of such a medium, to a different flowable medium, e.g. a flowable medium of a different color.
In most prior art screen printing machines operating in accordance with the principles described it has been necessary for such a color change to shut down the entire machine and to replace the entire applicator unit, for example, the applicator tube and applicator roller and to remove the screen, clean it and return it to the machine. The apparatus downtime as a result is relatively long.
In the European Patent Document EP-A2 277 481, it is known to effect the cleaning of the machine without requiring its disassembly. For this purpose, the medium distribution tube is supplied with flushing liquid which can be a solvent for the previously used flowable medium and which serves, when the screen rotates past this applicator, to clean the internal surfaces of the screen. A suction device is provided in addition to draw off the flushing liquid which is used. A disadvantage of this type of machine is that during the cleaning only the inner surface of the template or screen can be cleaned and that substantially all of the medium which may remain in the medium distribution tube must be forced through the latter and onto the screen or template or pattern and then flushed therefrom by the flushing liquid.
As a consequence, relatively large volumes of the flushing liquid must be used and the flushing operation is time consuming.
Furthermore, a uniform cleaning of the pattern or screen, because of the spacing and location of the suction openings is not always possible.
Another apparatus for this purpose is described in European patent publication EP - A1 91 716. This system utilizes two applicator tubes in a single applicator body which extend over the entire work width of the machine to provide a uniform application of the medium. The two medium-distribution tubes open into a single gap and are connected to a single medium-supply vessel. Since the two medium tubes are traversed in opposite directions by the respective portions of the medium the distribution of the medium over the width of application can be relatively uniform.
Swiss Patent 556,236 describes a rotatable doctor-blade casing with only a single applicator tube. After rotation of the doctor blade casing, the residual medium therein can be permitted to flow more or less completely back into the medium supply vessel.
Austrian Patent 360 950 describes an apparatus enabling the applicator width of a screen printing machine to be adjusted by adjustable lateral members. This publication also describes the use of level sensors which can control or monitor the depth of the bank of material immediately upstream of the location at which the flowable medium is transferred to the substrate.